The invention relates to a lockable motor assembly for use in a well bore.
The use of a downhole motor to drive a rotating tool, for example a milling tool, in a downhole assembly has well recognised advantages. Downhole motors available heretofore have, however, suffered from the disadvantage that the rotor of the motor cannot be locked to the stator of the motor. As a result, relative rotation of the elements of a tool assembly above and below the motor is possible. This renders impossible or at least complicates the accurate angular orientation of the components located below the motor.
If the components below the motor include a hydraulically settable packer or anchor, the fluid displacement required to set the packer or anchor is liable to drive the motor during the setting procedure, further complicating accurate angular orientation of the tool.
In certain applications, for example the drilling of a lateral well bore from a main well bore using a Whipstock to deflect a milling tool, accurate angular orientation of certain components (the whipstock in this case) is critical.
A prior art lockable motor assembly is disclosed in FR-A-2,332,412. This prior art motor assembly comprises a locking member which is initially fixed, by means of a shear pin, relative to a motor stator at a location axially spaced from the motor rotor. The arrangement is such that the rotor may freely rotate relative to the stator. In the event that the rotor is to be locked relative to the stator, a drop ball is released into the motor assembly and received on a shoulder within a bore of the locking member. The position of the drop ball within the bore of the locking member allows a fluid flow within the motor assembly to apply sufficient force on the locking member to shear the shear pin and move the locking member into engagement with the rotor. The rotor is thereby rotationally fixed relative to the stator. The prior art motor assembly does not comprise means for returning the locking member to a positon where the rotor is free to rotate relative to the stator.
A further prior art motor assembly is disclosed in U. S. Pat. No. 4,705,117. This prior art motor also comprises a rotor lockable relative to a stator by means of a locking member. During use of the prior art motor, the locking member is initially located so as to prevent rotation of the rotor relative to the stator. The locking member is rotationally fixed relative to the stator and secured to the rotor by means of a plurality of shear pins. In the event that the rotor is to be rotated relative to the stator, a drop ball is released into the motor assembly and received by the locking member. As a consequence, passageways defined in the locking member are closed by the drop ball allowing a fluid flow within the motor assembly to apply sufficient force on the locking member to shear the plurality of shear pins and move the locking member into a position whereby both the rotor and the locking member may rotate relative to the stator.
In accordance with the present invention a lockable motor assembly for use in a well bore comprises: the stator; a rotor rotatably mounted in the stator; a locking member movable axially of the motor between a first position in which the locking member is rotationally. fast with the stator and is in engagement with the rotor to prevent rotation of the rotor relative to the stator, and a second position in which one of the stator and the rotor is free to rotate relative to the locking member to permit relative rotation between the stator and the rotor; and holding means for holding the locking member in the first position and selectively releasable to permit the locking member to move to the second position; and pressure sensitive means for moving the locking member between the first and second positions; characterised in that the pressure sensitive means comprises a differential area piston provided by the locking member whereby fluid acting on one portion of the piston is sealed from fluid acting on another of the piston.
Preferably, the locking member when in its first position is rotationally fast with the rotor and when in its second position is spaced from the rotor to permit rotation of the rotor relative to the locking member and the stator. If the motor is of the type in which the rotor, in use, rotates about a fixed axis (if for example it is of the vane type, the turbine type or the positive displacement type), the locking member preferably has a non-circular profile which, when the locking member is in the first position, engages a complementary non-circular profile provided on the rotor. The non-circular profile on the locking member preferably takes the form of a projection which, when the locking member is in the first position, engages a complementary recess provided in the rotor. The non-circular profile is preferably provided by a plurality of splines.
If the motor is of the PDM type, the rotator will, in use, rotate about an axis which itself precesses around a circular path. Under these circumstances it is not strictly necessary for the locking member and the rotor to have complimentary inter-engaging non-circular profiles. As long as the locking member prevents precessional movement of the rotor axis the rotator will be locked against rotation. The locking member and the rotor may accordingly have mating circular profiles, or engage each other in some other way. However, even if the motor is of the PDM type the rotor may be locked by use of mating non-circular profiles on the locking member and the rotor.
Preferably, means sensitive to hydraulic pressure within the motor assembly are provided for releasing the holding means when the pressure within the motor assembly reaches a predetermined value. Preferably, the pressure sensitive means comprises a differential area piston provided by the locking member.
Preferably the holding means comprises one or more shear elements, for example one or more shear pins or a shear ring.
Preferably, means are provided for maintaining the locking member in the second position after it has been shifted from the first position to the second position.
Preferably, the locking member includes a through passage which provides fluid communication from the proximal end of the motor assembly to the input to the motor.
Preferably, if the motor is of the PDM type the rotor thereof will include a through passage which, when the locking member is in its first position. communicates with the through passage in the locking member to provide a fluid passage from the proximal end of the motor assembly to the distal end thereof. This fluid passage may conveniently be used to communicate fluid pressure to a packer, an anchor or other tool which is connected to the motor assembly and located below the motor assembly. If the motor is of a type (for example a vane type) which permits some flow through the motor even when the rotor is locked, the provision of a through passage in the rotor may not be necessary to set a packer or anchor, but none the less may be desirable since it will allow fluid to be pumped through the motor to perfom auxiliary function below the motor, e.g. bit cooling or cuttings removal.
If a packer or anchor is connected to the motor, the shear means may be designed to shear at a pressure higher than the setting pressure of the packer or anchor so that a complete assembly which includes the motor assembly and the packer/anchor may be run into a well bore, rotationally oriented, the packer/anchor set, and the shear means sheared to release the motor for rotation, all in a single trip. Preferably, a whipstock and a mill will be located between the motor and the packer/anchor so that after the packer/anchor has been set and the locking member has been moved to the second position to release the rotor, the motor can be operated to rotate the mill and form a window in the well casing.
Preferably, the motor is a PDM motor with directional drilling ability.
The preferred embodiment of the invention permits an assembly of one or more packers and/or anchors, a whipstock, one or more mills and/or bits, and a locked PDM motor to be run in to a well bore in a single trip. The assembly can be rotated to orient the whipstock correctly using appropriate orientation techniques. The pressure in the tubing string may then be increased to sequentially set the packer/anchor(s) and move the locking member to its second position thereby releasing the PDM motor rotor for rotation. The mill can then be sheared from the whipstock and mud flow increased to activate the motor and commence milling. Accordingly, a PDM powered whipstock milling assembly may be run in to a well, oriented, set, and activated to mill a window in a casing in a single trip. If the lead mill is of an appropriate type, for example a PDC bit designed to drill formation, the assembly may be used to drill to the required depth after it has broken through the casing. Accordingly all the steps necessary to drill a lateral may be completed in a single trip.